Light and Dark Spots

There's more going on here that you haven't explained yet. On x-ray images,
why are there light and dark spots?

The light and dark spots come from when you expose a photographic plate to
x-rays. The plate starts off whitish, and when x-rays strike the plate, that
section of the plate gets darker.

That means that different amounts of x-rays are reaching different parts of
the plate.

Uh-huh. And why should there be these differences? Try this out.

That makes sense and explains something else. Anytime there is more matter,
whether it's thicker or just has a higher density, fewer x-rays get out. Bones
appear lighter on the image because bone is denser than the rest of the body.

And in dental x-rays, metal fillings appear white, because the metal is very
dense. That's why they put a lead apron on you, to protect your body from the
x-rays.

If you're trying to x-ray a tooth, there is no need to have x-rays through
your entire body.